SICS motor handling

In SICS each motor is an object with a name. Motors may take commands which basically come in the form

motorname command ; example: D2HR list.

Most of these commands deal with the plethora of parameters which are associated with each motor. The syntax for manipulating variables is, again, simple.

Motorname parametername

will print the current value of the variable.

Motorname parametername newval

will set the parameter to the new value specified. A list of all parameters and their meanings is given below. The general principle behind this is that the actual (hardware) motor is kept as stupid as possible and all the intracacies of motor control are dealt with in software. Besides the parameter commands any motor understands these basic commands:

• Motorname list gives a listing of all motor parameters.
• Motorname reset resets the motor parameters to default values. This is software zero to 0.0 and software limits are reset to hardware limits.
• Motorname position prints the current position of the motor. All zero point and sign corrections are applied.
• Motorname hardposition prints the current position of the motor. No corrections are applied. Should read the same as the controller box.
• Motorname interest initiates automatic printing of any position change of the motor. This command is mainly interesting for implementors of status display clients.

The motor parameters

• HardLowerLim is the hardware lower limit. This is read from the motor controller and is identical to the limit switch welded to the instrument. Can usually not be changed.
• HardUpperLim is the hardware upper limit. This is read from the motor controller and is identical to the limit switch welded to the instrument. Can usually not be changed.
• SoftLowerLim is the software lower limit. This can be defined by the user in order to restrict instrument movement in special cases.
• SoftUpperLim is the software upper limit. This can be defined by the user in order to restrict instrument movement in special cases.
• SoftZero defines a software zero point for the motor. All further movements will be in respect to this zeropoint.
• Fixed can be greater then 0 for the motor being fixed and less then or equal to zero for the motor being movable.
• InterruptMode defines the interrupt to issue when the motor fails. Some motors are so critical for the operation of the instrument that all operations are to be stopped when there is a problem. Other are less critical. This criticallity is expressed in terms of interrupts, denoted by integers in the range 0 - 4 translating into the interrupts: continue, AbortOperation, AbortScan, AbortBatch and Halt. This parameter can usually only be set by managers.
• Precision denotes the precision to expect from the motor in positioning. Can usually only be set by managers.
• AccessCode specifies the level of user privilege necessary to operate the motor. Some motors are for adjustment only and can be harmful to move once the adjustment has been done. Others must be moved for the experiment. Values are 0 - 3 for internal, manager, user and spy. This parameter can only be changed by managers.
• Sign reverses the operating sense of the motor. For cases where electricians and not physicists have defined the operating sense of the motor. Usually a parameter not to be changed by ordinary users.
• failafter This is the number of consecutive failures of positioning operations this motor allows before it thinks that something is really broken and aborts the experiment.
• maxretry When a motor finishes driving, SICS checks if the desired position was reached. If the position read back from the motor is not within precision to the desired value, the motor is restarted. This is done at max maxretry times. After maxretry retries, the motor throws an error.
• ignorefault If this is bigger then 0, positioning faults from the motor will be ignored.

Motor Error Handling Concepts

As mechanical components motors are prone to errors. SICS knows about two different classes of motor errors:

HWFault

This is when there is a problem communicating with the motor, a limit is violated etc. SICS assumes that such errors are so grave that no fix is possible. If such a HWFault is detected a configurable interrupt (see parameter InterruptMode) is set which can be used by upper level code to act upon the problem.

HWPosFault

This is a positioning failure, i.e. The motor did not reach the desired position. Such a positioning problem can come from two sources:

• The positioning problem is reported by the motor driver. SICS then assumes that the driver has done something to solve the problem and promotes this problem to a HWFault.
• The motor driver reported no error and SICS figures out by itself, that the desired position has not been reached. SICS thinks that this is the case if the difference between the desired position and the position read from the motor controller is greater then the parameter precision. If SICS detects such a problem it tries to reposition the motor. This is done for the number of times specified through the parameter maxretries. If the position has not been reached after maxretries repositionings, a HWFault is assumed.

If SICS tries to drive an axis which is broken hardware damage may occur (and HAS occurred!). Now, SICS has no means to detect if the mispositioning of a motor is due to a concrete block in the path of the instrument or any other reason. What SICS can do though is to count how often a motor mispositions in sequence. This means SICS increments a mispositioning counter if it cannot drive a motor, if the motor is driven succesfully, the mispositioning counter is cleared. If the count of mispositionings becomes higher then the parameter failafter, SICS thinks that there is something really, really wrong and aborts the measurement and prints an error message containing the string: MOTOR ALARM.

There are some common pitfalls with this scheme:

You want upper level code to be signalled when your critical motor fails.

Solution: set the parameter interruptmode to something useful and check for the interrupt in upper level code.

SICS falsly reports mispositionings.

Solution: increase the precision parameter.

You know that a motor is broken, you cannot fix it, but you want to measure anyway.

Solution: increase the precision parameter, if SICS finds the positioning problem, increase maxretries, increase the failafter parameter. In the worst case set the ignorefault parameter to greater 0, this will prevent all motor alarms.